MINOR: clock: provide a function to automatically adjust now_offset

Right now there's no way to enforce a specific value of now_ms upon
startup in order to compensate for the time it takes to load a config,
specifically when dealing with the health check startup. For this we'd
need to force the now_offset value to compensate for the last known
value of the current date. This patch exposes a function to do exactly
this.

include/haproxy/clock.h

@@ -48,6 +48,7 @@ char *timeofday_as_iso_us(int pad);
 uint clock_report_idle(void);
 void clock_leaving_poll(int timeout, int interrupted);
 void clock_entering_poll(void);
+void clock_adjust_now_offset(void);
 
 static inline void clock_update_date(int max_wait, int interrupted)
 {

src/clock.c

@@ -304,6 +304,11 @@ void clock_init_process_date(void)
 	clock_update_date(0, 1);
 }
 
+void clock_adjust_now_offset(void)
+{
+	HA_ATOMIC_STORE(&now_offset, now_ns - tv_to_ns(&date));
+}
+
 /* must be called once per thread to initialize their thread-local variables.
  * Note that other threads might also be initializing and running in parallel.
  */